A number of flow measurement and detection arrangements have been developed over the years. These devices may be generally categorized as one-, two- and three-element devices. Single sensor devices can calculate fluid velocity, but are generally not suitable for determining fluid flow direction.
Two- and three-element devices generally operate on the principle of adding heat to a flowing fluid and measuring the heat transfer functions of sensors placed along the fluid flow stream. The difference between the heat transfer functions of the upstream and downstream sensors is used to calculate flow direction and velocity.
For example, U.S. Pat. No. 4,787,251 reports a two-element configuration. The configuration in U.S. Pat. No. 4,787,251 requires that both sensors produce a significant thermal wake. To produce these thermal wakes, both sensors are typically 30.degree.-100.degree. C. above that of the surrounding fluid temperature, a significant power requirement. The thermal wake is sensed by the downstream sensor. This two-element arrangement also requires thermally insulated flow sensors and greater complexity in the circuitry used to measure the electrical resistance or heat transfer functions of the thermal sensors. Use of the sensors as both thermal sensors and heating elements also results in unnecessary power dissipation in the measuring circuit. Additionally, the heat added by the downstream element is not utilized.
U.S. Pat. No. 3,196,679 describes a three-element configuration which uses a heated element situated between an upstream and a downstream thermal sensor. This three-element configuration requires a high heat source and relies on the mechanical movement of spring-type thermal sensors. Because spring-type thermal sensors are not appropriate for detecting small temperature differentials of a surrounding gas flow, this configuration is not suitable for use in measuring low volume gas flow.
Thus, there exists a need for an inexpensive, reliable, and low-power fluid flow direction and velocity monitor for use in laboratories, hospitals and other ventilation applications where airborne contaminants must be isolated within or outside of a controlled space. Reliable fluid flow direction monitors may be used to maintain minimal pressure differentials between the controlled and non-controlled spaces. Reliable detection of minimal pressure differentials allows for the use of lower volume air supply systems, therefore, reducing energy requirements.